Light-emitting device and method of manufacturing the same

ABSTRACT

Provided is a light-emitting device and a method of manufacturing the same. The light-emitting device includes a substrate having at least one protruded portion with a curved surface in which a consistent defect density and uniform stress distribution can be obtained even when the growth of the semiconductor crystal layer and the forming of the light-emitting device are completed. In addition, the light-emitting device has a high the light extraction efficiency for extracting light generated at an electroluminescense layer externally.

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Application No.2003-73442 filed on Oct. 21, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

1. Field of the Invention

The present invention relates to a light-emitting device, and moreparticularly, to a high efficiency light-emitting device with improvedlight extraction efficiency and good defect density control and stressdistribution control and, in which, a substrate limits a surface crystalorientation.

2. Description of the Related Art

In general, light-emitting devices include laser diodes (LD) and lightemitting diodes (LED), and LEDs use properties of compoundsemiconductors to transmit a signal, which is electric energy convertedinto an infra-red light, visible light, or other forms of light. Theconverting of electric energy into light can be categorized intotemperature radiation and luminescence. Photo luminescence caused by theexcitation of light, a cathode luminescence caused by the irradiation ofx-ray or an electron beam, and electroluminescence (EL) are all types ofluminescence. An LED is one kind of an EL and currently LEDs using groupIII-V compound semiconductors are widely used.

Group III nitride compound semiconductors are direct transitionsemiconductors, and are widely used in light-emitting devices such asLEDs and LDs since it is possible to obtain stable operation at a hightemperature than devices that use other semiconductors. In general, theGroup III nitride compound semiconductors use sapphire (Al₂O₃) as asubstrate and are formed on top of the substrate.

FIG. 1 is a cross-sectional view of in general Group III nitridecompound semiconductor, including a sapphire substrate. A n-GaN layer12, an active layer 13, a p-GaN layer 14, and a p-type electrode layer15 are sequentially formed on a sapphire substrate 11. In addition, ann-type electrode layer 16 is formed on the n-GaN layer 12 where theactive layer 13 is not formed. In a general LED, the most importantissue is how efficiently can the light, which is created at an internalactive layer, be extracted externally.

To efficiently extract light created in the longitudinal direction ofthe sapphire substrate and active layer, efforts to form transparentelectrodes or reflective layers have been made. However, a large amountof light, which is created at the active layer, is transmitted in alatitudinal direction. Therefore to extract the light in a longitudinaldirection, various methods such as forming the side walls of thestructure of accumulative layers of a semiconductor device to have apredetermined angle, and forming side walls composed of reflectivematerial have been made, but this caused problems in the manufacturingprocess and increased costs. Furthermore, to increase the light emittingability of Group III nitride compound semiconductor light-emittingdevices that use a sapphire substrate, a device with a flip chip-type isadopted and the light extraction efficiency is at approximately 40% dueto the difference in diffraction rates between the GaN and sapphiresubstrate.

Recently, as shown in FIG. 2 a, an LED structure in which an unevenstructure is formed by processing the surface of a sapphire substrate 21and forming semiconductor crystal layers, which include active layers,on top of the substrate has been introduced. Such a structure forms adiffraction rate interface having an uneven surface under the activelayer 22 and enables the external extraction of a portion of the lightwhich fades out within the device.

In addition, when forming a Group III nitride compound semiconductor onthe sapphire substrate 21, a dislocation occurs due to the miss fit ofthe sapphire substrate 21 and the lattice parameters of a Group IIInitride compound semiconductor. To prevent this, as shown in FIG. 2 b,the sapphire substrate 21 has an uneven surface and a GaN layer 23 isformed on top. FIG. 2 c schematically illustrates a process of formingan LED on top of a sapphire substrate which has such an unevenstructure. When forming the GaN layer 23 on top of the sapphiresubstrate 21 which has an uneven structure as shown in FIG. 2 c-(a), GaNfacets are grown 24 from the top and each side portion of the unevenstructure, as shown in and 2 c-(b). Then a planarized GaN layer 23 canbe obtained as shown in FIG. 2 c-(c). FIG. 2 c-(d) illustrates thecompletion of a light emitting diode, in which an active layer 22, etc.are on top of the planarized GaN layer 23.

This process has a disadvantage in that when growing the semiconductorcrystal layer using such a patterned sapphire substrate (PSS), sinceplanarization is carried out after facet growth is performed on thepattern, regrowth has to be done to a sufficient thickness to performplanarization.

In addition, a structure is disclosed (No. WO2001-69663), in which astep difference is formed, group III nitride compound semiconductors aregrown on the top surface and side portions of the step difference and apiercing phase is prevented. However, a disadvantage is that a void isformed in the lower portion of the step difference and to planarize thegrowth layer group III nitride compound semiconductors have to be formedrelatively thick.

When regrowing the semiconductor on the sapphire substrate, an ELOG anda PENDEO method are used to reduce the defect density. However, in thecase of the ELOG method a separate mask layer is needed, and in the caseof the PENDEO method, a void is formed on the interface portion of thesubstrate resulting in a decrease in light extraction efficiency.

SUMMARY OF THE INVENTION

The present invention provides a light-emitting device and a method ofmanufacturing the same. The light-emitting device includes a substratehaving at least one protruded portion with a curved surface in which aconsistent defect density and uniform stress distribution can beobtained even when the growth of the semiconductor crystal layer and theforming of the light-emitting device are completed. In addition, thelight-emitting device has a high the light extraction efficiency forextracting light generated at a electroluminescense layer externally.

According to an aspect of the present invention, there is provided alight-emitting device including a substrate having at least oneprotruded portion with a curved surface, the crystal surfaceorientations of the at least one protruded portion are different fromgrowth directions of a group III nitride compound semiconductor formedon the at least one protruded portion, and a plurality of semiconductorcrystal layers comprising a plurality of active layers and electrodesformed on a portion of the substrate.

In the present invention the curvatures at each point of the surface ofthe protruded portions is greater than 0.

In the present invention each surface of the protruded portions has adifferent crystal orientation from a (0001) surface.

In the present invention the substrate is composed of sapphire ormaterial including Si.

In the present invention an n-GaN layer is formed on the substrate, anactive layer, a p-GaN layer, and a p-type electrode layer are formedsequentially on a portion of the n-GaN layer, and an n-type electrodelayer is formed on a portion of the n-GaN layer where the active layeris not formed.

According to another aspect of the present invention, there is provideda method of manufacturing a light-emitting device includes forming atleast one protruded portion with a curved surface on a planarizedsubstrate, and forming semiconductor crystal layers including activelayers, on the substrate.

In the present invention, forming the at least one protruded portionincludes patterning a photo resist formed on the substrate, hard bakingthe photo resist and the substrate, and etching the surface of thesubstrate, thereby forming at least one protruded portion.

In the present invention when etching the substrate surface, an etchinggas is a Cl gas selected from the group consisting of Cl₂, BCl₃, HCl,CCl₄, and SiCl₄.

According to another aspect of the present invention, there is provideda method of manufacturing a light-emitting device including forming atleast one protruded portion with a curved surface on a substrate, andgrowing group III nitride compound semiconductor crystal layers from thesubstrate surface between the protruded portions until the surface ofthe protruded portions is covered.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a cross-sectional view of a general Group III nitride compoundsemiconductor using a sapphire substrate;

FIGS. 2 a and 2 b illustrate a general structure and process of forminga Group III nitride compound semiconductor on an eneven sapphiresubstrate;

FIG. 2 c illustrates a general process of forming an LED on a substratewhich has an uneven surface;

FIG. 3 a is a cross-sectional view of a substrate of a light-emittingdevice according to an embodiment of the present invention;

FIG. 3 b is a SEM picture of the surface of the substrate of FIG. 3 a;

FIG. 3 c is a cross-sectional view of a flip chip-type light-emittingdevice, which includes a substrate in which curved surface typeprotrusion portions are formed according to the present invention;

FIGS. 4 a through 4 d are cross-sectional views illustrating a method ofmanufacturing a light-emitting device according to an embodiment of thepresent invention;

FIG. 5 a is a SEM picture illustrating the process of coating a GaNlayer on top of the sapphire substrate, which ha a curved surface typeprotruded portions and planarizing the GaN layer according to thepresent invention, and FIG. 5 b is a SEM picture illustrating theprocess of coating and planarizing a GaN layer on top of the sapphiresubstrate which has an uneven surface that has a planarized top; and

FIG. 6 is a diagram to compare the light extraction power of thelight-emitting device according the present invention with that of priorart.

DETAILED DESCRIPTION OF THE INVENTION

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings.

FIG. 3 a is a cross-sectional view of a substrate of a light-emittingdevice which might include sapphire or Si according to an embodiment ofthe present invention. As shown in FIG. 3 a, smoothly curved protrusions32 are formed on the surface of a substrate 31. Such protrusions 32 aredifferent from the uneven structure of the general substrate shown inFIGS. 2 a through 2 c. In other words, the top portion and side portionof the uneven structure of FIGS. 2 a through 2 c are discriminated sincethe top portion and side portion are each flat and the side portions areslanted at a predetermined angle with respect to the surface of thesubstrate 21.

However, the protruded portions 32 formed on the surface of thesubstrate of FIG. 3 a has a curved surface, and thus there is notdistinction between the upper portion and side portion, resulting in asurface in which a planarized surface does not exist. Therefore, thecurvatures of each portion of the protruded portions 32 are greaterrthan 0. Except where the protruded portion 32 and the main part of thesubstrate 31 meet, a corner does not exist. Therefore, crystalorientations of the surfaces of the protruded portions 32 of thesubstrate 31 are different from crystal growth directions (c axis) ofthe Group III nitride compound semiconductors, which are formed on topof the substrate 31. In other words, the surfaces of the curved surfacetype protruded portions 32 are formed of a crystal growth surfacesdifferent from a (0001) surface. Therefore, the growth of group IIInitride compound semiconductors does not frequently occur on the surfaceof the protruded portions 32.

FIG. 3 b is a SEM picture of the surface of the substrate 31 of FIG. 3a. The protruded portions 32 use hemispheres. All protruded portions 32on the surface of the substrate may have identical sizes and forms, butembodiments of the invention are not limited to this, and the size, formand curvature of each portion of the protruded portions 32 may differslightly. For example, the curvature of the lower portion of theprotruded portions 32 may be greater than the curvature of the upperportion or vise versa. In addition, the entire form of the protrudedportions 32 may have curved surfaces, be hemispheres, form a stripedpattern or have a bent horseshoe shape. Furthermore, there is no limitto the arrangement of the protruded portions so that the protrudedportions may have a regular spaced arrangement such as a latticestructure or an irregular spaced arrangement.

The substrate 31 of the light-emitting device according to an embodimentof the present invention is not limited to a sapphire substrate, and anysubstrate that grows Group III nitride compound semiconductors such asSi, SiC etc. may also be used.

FIG. 3 c is a cross-sectional view of a flip-chip-type light-emittingdevice including the substrate 31 according to an embodiment of thepresent invention. Referring to FIG. 3 c, an n-GaN layer 33 is formed onthe substrate 31 and an active layer 34, a p-GaN layer 35, and a p-typeelectrode layer 36 are sequentially formed on a portion of the n-GaNlayer 33. In addition, an n-type electrode layer 37 is formed on aportion of the n-GaN layer 33 where the active layer 34 is not formed.The structure of the light emitting device, aside from the substrate 31,is not much different from that of the group III nitride compoundsemiconductor light-emitting device. A group III nitride compoundsemiconductor formed on the substrate 31 is not limited to GaN, and mayalso include secondary molecules such as AlN or InN, other tertiarymolecules and quadruple molecules.

A method of manufacturing a light-emitting device according to anembodiment of the present invention will be described below. Thefollowing is a process of forming a plurality of curved surface typeprotruded portions on the surface of a substrate.

First, a photo resist on the planarized substrate is patterned. Thepatterning is carried out using a general photolithography method, andthe thickness of the photo resist depends on the target value of theetching depth of the substrate. For example, when the etching depth ofthe sapphire substrate is approximately 1.2 μm the thickness of thephoto resist can be approximately 2 μm.

Next, hard baking is performed at a temperature of approximately 110° C.

In addition, when etching the sapphire substrate, a general reactiveion-etching method is used. Etching gas, pressure, and power aresuitably adjusted to form a protruded portion of the substrate. In thepresent embodiment Cl₂/BCl₃ is used as an etching gas, with a pressureof 3 mTorr, and a power of 800 W. For example, the etching gas can beselected from the Cl group of Cl₂, BCl₃, HCl, CCl₄, and SiCl₄ etc.Furthermore, the pressure can be between a few mTorr and tens of mTorr,depending on the etching gas, and is preferably 1˜40 mTorr.

A plurality of protruded portions can be formed on the substrate surfaceby the above-described process. Then an n-GaN layer, an active layer, ap-GaN layer, a p-type electrode layer, and n-type electrode layer, areformed on the substrate. In the light-emitting device according to thepresent embodiment, the compound semiconductor layers formed on thesubstrate 31 are not largely different from those of general lightemitting devices and the manufacturing process can be understand bythose skilled in the art. However, when forming the light-emittingdevice on the substrate surface, with the protruded portions, a separatemask layer is not needed.

FIGS. 4 a through 4 d schematically illustrate a method of manufacturingthe light-emitting device according to an embodiment of the presentinvention. Referring to FIGS. 4 a and 4 b, the surface of the planarizedsubstrate 31 is etched, thereby forming a sapphire substrate 31 whichhas protruded portions 32 with curved surfaces. Referring to FIG. 4 c,the GaN layer 33 is grown on the substrate. Referring to FIG. 4 d, theGaN layer 33 is grown to a predetermined thickness and its surface isplanarized. FIG. 3 c illustrates a completed flip-chip typelight-emitting device in which other active layers 34 and electrodelayers 36 and 36 are all formed on top of the planarized n-GaN layer 33of FIG. 4 c. A void is not formed at an interface between the substrate31 and the GaN layer 33. Since the growth process for active layerscarried out after the forming of the GaN layers 33 is well described inthe prior art, it will not be mentioned here.

The method of manufacturing a light-emitting device illustrated in FIGS.4 a through 4 d is different from the method forming a light-emittingdevice on a substrate that has an uneven surface as shown in FIG. 2 c.When obtaining a GaN planarizied layer 32 in a light-emitting deviceaccording to an embodiment of the present invention, the GaN does notgrow facets as in the prior art and the thickness of the GaN layer 32for obtaining a planarized layer is relatively thin. In addition, in theprior art, an epitaxial growth occurs, but in the present invention,growth of the GaN layer 33 does not easily occur on the surface of theprotruded portion 32.

In the present embodiment, for the crystal growth direction of the GroupIII nitride compound semiconductor formed on the top of the surface ofthe protruded portions 32 to have a plurality of crystal orientations,the protruded portions 32 must have curved surfaces. Therefore, thegrowth of the group III nitride compound semiconductor starts on thesurface of the substrate 31 between the protruded portions 32, that is,a planarized portion, and as the thickness of the group III nitridecompound semiconductor increases, the side and top portions of theprotruded portions 32 become covered.

This is confirmed in FIGS. 5 a and 5 b. FIG. 5 a is a SEM imageillustrating a process of coating the GaN layer 33 on the substrate 31with the protruded portions 32 and FIG. 5 b is a SEM image illustratinga process of coating the GaN layer 33 on the general substrate 21 withan uneven surface.

The uneven structure, which has a planar surface and protruded portions32 with curved surfaces formed on the surface of the substrate 31 aremanufactured at an identical height. When viewing the SEM imagesillustrating the GaN layer 33 being formed on the two substrates 31 inidentical conditions, it is obvious that there is a difference betweenthe present invention illustrated and the prior art in FIGS. 5 a-(b) and5 b-(b).

In other words, in FIG. 5 a-(b) the planarization of the substrate 31 iscarried out by the GaN layer 33, except on the top portion of theprotruded portions 32. On the other hand, in FIG. 5 b-(b), facet growthof the GaN occurs on the top and side surfaces of the uneven portion andthe degree of planarization is very low. In addition, referring to FIG.5 a-(c), on top of the substrate 31, GaN is coated and completeplanarization is achieved. However, referring FIG. 5 b-(c),planarization is carried out on top of the uneven surface but completeplanarization is not achieved between each uneven surface.

FIG. 6 is a graph comparing the light extraction of the light-emittingdevice according to the present invention with that of prior art. Aillustrates the light-emitting device which is formed on the generalplanarized substrate 11 shown in FIG. 1. B illustrates thelight-emitting device formed on the general substrate 21 having theuneven surface shown in FIG. 2 d. C illustrates a light-emitting device,formed on the substrate 31 having protruded portions 32 with curvedsurfaces according to the present invention, shown in FIG. 3 c.

Referring to FIG. 6, the light extraction in case B in which thelight-emitting device is formed on the substrate 21 with an unevensurface is 50% greater than the case of A in which the light-emittingdevice is formed on the substrate 11 with the planar surface. The lightextraction in case C in which the light-emitting device is formed on thesubstrate 31, with the protruded portions 32, with the curved surfacesis greater than 60% more than in case A. In addition, light extractionin case C is approximately 10% than in case B. This is because in thesubstrate 31 with the protruded portions 32, the semiconductor crystallayer forms an optical lens which changes the light path and reduces thedefect density of the growing semiconductor crystal layer.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims. Forexample, the protruded portions of the light-emitting device accordingto embodiments of the present invention are different from the crystalgrowth directions of the Group III nitride compound semiconductor to beformed on the substrate, and may be hemispheres, stripes, horse shoeshapes etc., and the arrangement can include regular and irregulararrangements.

According to embodiments of the present invention, when forming alight-emitting device which includes an electroluminescence layer on topof a substrate having protruded portions with curved surfaces,planarization is carried out efficiently and consistent defect densitycontrol and control of stress distribution is easily attained, even whenthe growth and light-emitting device of the semiconductor crystal layerare complete, and, as a result, can increase light extraction efficiencyof the light which is generated at the electroluminescence layer aredirected toward the outside of the light-emitting device.

1. A light-emitting device comprising: a substrate having at least oneprotruded portion with a curved surface, the crystal surfaceorientations of the at least one protruded portion are different fromgrowth directions of a group III nitride compound semiconductor formedon the at least one protruded portion; and a plurality of semiconductorcrystal layers comprising a plurality of active layers and electrodesformed on a portion of the substrate.
 2. The device of claim 1, whereincurvatures at each point of the surface of the protruded portions isgreater than
 0. 3. The device of claim 2, wherein each surface of theprotruded portions has a different crystal orientation from a (0001)surface.
 4. The device of claim 1, wherein the protruded portions arehemispheres or stripes.
 5. The device of claim 1, wherein the substrateis composed of sapphire or material including Si.
 6. The device of claim1, wherein an n-GaN layer is formed on the substrate, an active layer, ap-GaN layer, and a p-type electrode layer are formed sequentially on aportion of the n-GaN layer, and an n-type electrode layer is formed on aportion of the n-GaN layer where the active layer is not formed.
 7. Amethod of manufacturing a light-emitting device comprising: forming atleast one protruded portion with a curved surface on a planarizedsubstrate; and forming semiconductor crystal layers including activelayers, on the substrate.
 8. The method of claim 7 further comprising,before the forming the at least one protruded portion: patterning aphoto resist formed on the substrate; hard baking the photo resist andthe substrate; and etching the surface of the substrate, thereby formingat least one protruded portion.
 9. The method of claim 8, wherein, whenetching the substrate surface, an etching gas is a Cl gas selected fromthe group consisting of Cl₂, BCl₃, HCl, CCl₄, and SiCl₄.
 10. A method ofmanufacturing a light-emitting device comprising: forming at least oneprotruded portion with a curved surface on a substrate; and growinggroup III nitride compound semiconductor crystal layers from thesubstrate surface between the protruded portions until the surface ofthe protruded portions is covered.
 11. The method of claim 10, whereineach surface of the protruded portions has a different crystal directionfrom that of a (0001) surface.
 12. The method of claim 10, wherein thegrowing is epitaxial growing.